This invention relates to computerized axial tomographic analysis and, more particularly, to the determination of an atomic number image for a sample, such as a core sample from a borehole, and to the determination of a density image for the sample that is corrected for the effects of atomic composition of the sample by such analysis.
Atomic number imaging by a computerized axial tomographic scanner (hereinafter referred to as "CAT") is based on the fact that X-ray attenuation depends both on the density and the chemical composition of a material. Between mean energies of 80 keV and 1 MeV X-rays interact with matter predominantly by Compton scattering which is dependent on electron density. For X-ray energies below a mean energy of 80 keV, photoelectric absorption becomes important; this type of interaction is strongly dependent on atomic number. If the attenuation coefficients are measured at two X-ray energies, one in the Compton region and one in the photoelectric region, separate images of density and effective atomic number can be obtained. The prior art has employed preimaging methods involving hardware modifications, such as split-energy detectors or pulse-height counting, and extensive numerical calculation from the raw data. These hardware modifications are expensive to implement and maintain and require software modifications that are system dependent. A typical CAT has 720 detectors, but it may have as many as 1440 detectors. If split-energy detectors are used, 1440 electronic preamplifiers would be required to gather the data collected by the 720 split-energy detectors. If pulse-height counting is employed, a single channel analyzer would be required for each of the 720 channels. In either case the initial cost is high and the maintenance required is extensive. The numerical calculations required with the prior art techniques, such as that disclosed in Alvarez et al., U.S. Pat. No. 4,029,963, generally require solving nonlinear integral equations; these solutions are complex, time consuming and cannot be accomplished in real time.
In addition, prior art CAT systems have not corrected the density image of unknown materials for the effects of atomic composition, because of the complexity and expenses associated with split-energy detectors and pulse-height counting as discussed hereinabove.
Therefore, it is an object of the present invention to provide an atomic number imaging technique that simplifies the calculations required and eliminates the need for specialized hardware and software.
It is a further object of the present invention to provide a method of obtaining a density image of an unknown material that is corrected for the effects of atomic composition of the unknown material.